A standard turbine has a cantilevered impeller and a spur transmission comprising a transmission housing, a small-diameter pinion fixed rotationally to the impeller, and an output spur gear meshing with the pinion and rotatable on the housing about an axis parallel to the impeller axis. Normally the impeller and the pinion are carried on an output shaft that itself is supported in the housing in a bearing which is arranged between the impeller and the pinion. Such a bearing takes up considerable space and is a difficult item to manufacture to the necessary tolerances. In addition for a given weight and mass of the impeller only a limited range of constructions are possible to influence the critical speed.
The result is that such turbines must be built to fairly limited standards. The known turbines, in particular radial-throughput ones, are run in a supercritical range at very high speeds so that an acceptable service life of the spur gearing can only be obtained by mounting the output shaft on a bearing that is both cooled and lubricated by oil. Oil-lubricated bearings produce considerable friction at high speeds, which friction negatively affects turbine performance and requires an expensive oil-pumping and -cooling arrangement.